1. Field of the Invention
The present invention relates to a technique of coating a cathode electron emitter layer a FED (Field Emission Display), and particularly relates to a method for manufacturing a cathode electron emitter layer patterned via a patterned carbon nanotube process.
2. Background of the Invention
A FED is a device that utilizes a cathode electron emitter to generate surrounding electrons within an electric field; the electrons excite the phosphors coated on an anode for lighting. The FED is lightweight, thin and flimsy; the effective area thereof is variable to meet requirements but without the view angle problems that occur in a flat LCD (Liquid Crystal Display).
FIG. 1 shows a conventional FED 1a including a unit 5a within an anode 3a and a cathode 4a disposed therein, and a rib 53a arranged therebetween for separating the anode 3a from the cathode 4a and support. The anode 3a includes an anode glass substrate 31a, an anode conductive layer 32a, and a phosphors layer 33a arranged sequentially. The cathode 4a includes a cathode glass substrate 41a, a cathode electrode layer 42a, and a cathode electron emitter layer 43a arranged sequentially. The rib 53a connects the anode 3a and the cathode 4a, and a vacuum is accordingly formed therein. The cathode electron emitter layer 43a generates electrons for emission onto the phosphors layer 33a to produce light via an additional electric field; in addition, the rib 53a must be made of a insulating material to prevent conduction between the anode 3a and the cathode 4a while in the additional electric field. The additional electric field has a strength that depends on and is directly proportional to a voltage over the anode 3a and the cathode 4a, but is inversely proportional to a distance between the anode 3a and the cathode 4a. Therefore, thickness uniformity of each layer of the anode 3a and the cathode 4a and even height of the rib 53a decide a luminance uniformity of the conventional FED 1a. 
Iijima refers to a new material of carbon nanotube in 1991 (Nature 354, 56 (1991)); the new material has high aspect ratio, height mechanical intensity, high chemical resistance, high abrasion resistance, low threshold electric field, and other similar characteristics. The new material is adopted for field emission electrons and researched generally (Science 269, p1550 (1995); SID'98 Digest, P1052 (1998); SID'01 Digest, p316 (2001)). The field emission electrons are generated by escape from a material surface to be free electrons; a material surrounds in a high electric field to reduce an energy barrier thereof, electrons in the material then escape from the material surface by the quantum-mechanical tunneling effect (J. Appl. Phys. 39,7, pp 3504–3504 (1968)). An electric current accompanying the field emission electrons can be improved via the material with a low function, and the field emission electrons are generated via the additional electrical field, but not a heat source, so that devices equipped with the field emission electrons are called cold cathodes and are generally applied to a cathode electron emitter layer of a FED.
Manufacturing a cathode electron emitter layer of a FED includes at least two methods. One method is a chemical vapor deposition (CVD) process for depositing carbon atoms on a cathode substrate to be carbon nanotubes. A metal catalyst film is patterned first, and the carbon nanotubes grows from the metal catalyst film by the chemical vapor deposition process. Although the carbon nanotubes can be manufactured stably, each with a uniform length, the metal catalyst film is maintained still, and this will affect field emission electrons efficiency and further needs a surface treatment to increase the efficiency thereby. In addition, the CVD process is expensive and may restrict a size of the FED to less than 20 inches. Another method of a screen printing process can reduce costs thereof (disclosed in Taiwan Patent No. 502395); however, an electron emitter layer 60 (illustrated in FIG. 2) fabricated by a screen printing process still has some problems to be resolved. First, a paste comprising carbon nanotubes usually includes a viscosity above 100,000 cPs (centi-poise) to maintain a precision and a figure of a printing pattern. Second, for mating with a thickness of an emulsion and a fabric structure of a screen plate, the printing pattern has a size of more than 70 micrometers (μm) and meets high-resolution requirements with difficulty. Furthermore, the printing pattern has a thickness of 10 micrometers (μm) at least; the screen plate includes meshes, the thickness of which varies from 4 to 8 micrometers (μm) to influence a uniformity of the FED. Third, the aspect ratio of each of the nanotubes 62 is high enough (usually more than 40) to spread in the paste, but length of each nanotube 62 is restrictive due to the second reason, which is that the carbon nanotubes 62 still are easily buried in the paste, such as binders or conductive materials 61, after a sintering process, and the efficiency of electron generation is thereby decreased. For example, the electron emitter layer 60 fabricated by the screen printing process provides an electric current less than 10 mini amperes per square centimeter (mA/cm2) under an electric field of less than 4 volts per micrometer (V/micrometer (μm)).
Hence, an improvement over the prior art is required to overcome the disadvantages thereof.
The inventor utilizes a nanotube spray disclosed in T.W. Application No. 92131590, applied with a negative photoresist mask, to make a cathode electron emitter layer with high resolution according to the present invention. First, the negative photoresist mask is made of macromolecule polyester-polymer materials and negative photoresist for manufacturing a patterned electron emitter layer with high resolution; second, the negative photoresist mask and some carbon nanotubes disposed thereon can be removed by a sintering process simultaneously; third, the carbon nanotubes distribute uniformly by a spraying method for increasing efficiency of electron generation.